Method of case-hardening iron articles



Patented Apr. 1935 v UNITED STATES PATENT OFFICE amass METHOD or CASE-HABDENING moN narrows Walter Beck, Frankfort-on-the-Maln, and Klaus Bonath, Cronberg in Taunus, Germany, assignors to Deutsche Golda Silber Scheldeanstalt. vormals er, Frankiort-on-the- Main, Germany, a corporation No Drawing.- Application at, 29, 1931. Serial No. 54l,108. In Germany June i, 1930 6 Claims. (01. uni-15):

This invention relates to iron, particularly to treatment thereof, and more especially to that kind of treatment which is generally called cementation or case-hardening treatment.

Those skilled in the art of treating bodies composed entirely, or largely, of elementary iron know that there are certain forms of iron, generally known as crucible or tool steel, which may be hardened and tempered by well known processes, such as, heating followed by sudden cooling and partial reheating; This process causes a change, throughout the entire bodytreated, so, it might be said, that the hardening and tempering extend throughout the entire body. 1 Y

It is also known, by those skilled in the treatment of bodies composed entirely, or largely,,of elementary iron, that forms of iron, other than 'those known as tool or crucible steel, may be altered, more or less deeply, so as to present a comparatively-hard surface or shell, or in the case of extremely thin bodies, throughout the entire substance, by proper treatment. This proper treatment isv known as cementation or case-hardening. It may be, that the word cementation applies strictly to a process in which the iron is treated by being surrounded by a powder in which fusion does not take place, but as the term is commonly applied to processes in which fusion of the substance, used to treat the iron,

takes place, the term will be used hereinafter as synonomous with case-hardening, andeach will be considered to identify a process in which bodies,

composed largely of elementary iron which will not be hardened by the ordinary process by which tool steel is hardened and tempered, are hardened by a process in which the iron is subjected in a heated state to the action of various substances, compounds, and mixtures thereof to produce a more or less deep hardened surface, shell or casing thereon. Heretofore, iron has been case-hardened by heating it while in contact with carbonaceous nitrogenous substances, such as, for instance, leather chips. It has also been accomplished by the use of potassium ferrocyanide. Probably, the most used cementation or case-hardening substances, at the present time, is alkali or alkalineearth cyanides, especially sodium cyanide. A1- kali or alkaline-earth cyanldes, such as sodium cyanide, may be used'as a cementation or casehardening substance in various ways, but the most generally used method, at the present time, is to dip or immerse the heated iron-into molten mass or bath of the cyanide. 7 It may be that the action oi a cyanide bath on heated-iron is not exactly understood, but it is believed that a decomposition of the cyanide takes place, and an absorption or combination of the iron with the carbon of the cyanide and perhaps with the nitrogen takes place with the result that a substance similar to tool steel is formed as a layer or casing on the surface of the body and extending to some distance therein,'depending upon the concentration of the bath used, the heat employed, and the length of treatment, so that in some cases, especially in the case of very thin bodies, the action may extend completely through the bodyi I It is known that a case-hardening bath of the alkali or alkaline-earth cyanides ceases to caseharden with its original rapidity or even at all after a certain time, but before the bath is free of unaltered cyanide, that is, before all of the cyanide has been decomposed.

Careful investigation is believed to show that 20 during the case-hardening process the cyanides give rise to decomposition products, which remain in the bath. and are believed to be the oxide and carbonate of the metal or base of the cyanide. It is believed, that careful investigation has shown that the decompomtion products of the cyanide, such as, the oxide and the carbonate act to prevent further decomposition of the unaltered, original cyanide remaining in the bath. It is believed that this action is due to the factthat the 4 oxide and carbonate dissociate, and by causing a high concentration of the dissociated metal or base in the bath prevents the dimociation of the original cyanide and so prevents the case-hardening action of the cyanide. In short, the 'decompodtion products of the cyanide remaining in the bath act as stabilizers, or cyanide antidissociation substances. Investigation has fur-. ther shown that the stabilizing or anti-dissociation action of the decomposition products of the cyanide toward the original unaltered cyanide increases as the alkalinity of the decomposition products increases and also as their extent of dissociation increases.

A principal object of this invention, taking advantage of the hereinbefore mentioned discoveries and proceeding upon the basis thereof, is to provide a method: whereby a case-hardening bath may be prevented from arriving at he condition where there is still unaltered cyanide present therein, and it fails to case-harden iron bodies, or to case-harden themto the depth and at the depth and at the speedat which it originally did; or to revive a cyanide bath after having arrived at such condition.

A further object of the invention is, in general, to regulate the rate of eifective case-hardening decomposition of a cyanide'bath, to the end that the activity may be either increased or decreased or maintained substantially constant.

A further object of the invention is the provision of a mixture of substances, including a cyanide, which by periodic additions of a further mixture, compounded in accordance with this invention, will furnish a continuously acting casehardening bath, operating substantially uniformly, at the desired activity rate.

With the facts and the results of the herein mentioned investigations, and with the objects as hereinbefore stated, in mind, search was made for a substance or substances which could be added to a case-hardening bath as originally made up, or as it became exhausted, which would act as,

what might be called, an anti-stabilizer, that is, a substance which would-act reversely or in an the dissociation of the cyanide.

Investigation showed that the stabilizing anti-dissociation properties or these decomposition products may be compensated for or neutral- ;ized by the addition to a cyanide bath of certain compounds. Salts of the base of the cyanide may be used but the most emcacious compounds acting as anti-stabilizers, or compensating, or neutralizing substances have been found to be salts or bases more feebly electro-positive than the base of the cyanide used in the bath. If an alkali cyanide, such as sodium cyanide, is used as the active case-hardening material, compounds, for

instance, of the bases barium, strontium, calcium, cerium, and magnesium introduced. into the bath, produce an anti-stabilizer or neutralizing action, so that the dissociation or decomposition products of the original cyanide no longer effectively prevent further decomposition of unaltered cyanide. The eifectiveness of the compound added seems to be measured by its alkalinity. The less alkaline the compound, the greater its effect in neutralizing the stabilizing effect of the decomposition products or the cyanide. Further investigation has led to the knowledge that the best form, in which to use the neutralizing compounds, is in the form of those salts having a low melting point, such as chlorides.

With the results, now available by the discoveries, it is possible to quite fully regulate a case-hardening bath. The stabilising action of thedecomposition products may be increased,

undecomposed cyanide would be accentuated or the bath would have a reduced activity.

The stabilizing action of the decomposition products can be exactly compensated. This would best be done by adding a chloride of one I of the bases more feebly electro-positive than the base of the cyanide used, for instance, it the cyanide used is the sodium salt, then strontium chloride may be used as an anti-stabilizer. It should be used in such quantity that the stabilizing products of the decomposition products are substantially compensated.

If a considerable decrease of the stabilizing action of the decomposition is desired,then a sufflcient quantity of a salt, of a base more feebly electro-positive than the base of the cyanide used, would be added in a quantity sufficient to obtain the necessary accelerated case-hardening action,

for instance, if the cyanide used for case-hardening was the sodium salt, then strontium chloride, if added in sufllcient quantity would give the de-- sired accelerated action of the cyanide.

Not only may the action of the stabilizing decomposition products be increased, compensated, or decreased by the addition of a single substance to the bath but combinations of substances within the classes given can be so made as to regulate the case-hardening activity within quite narrow limits, for instance, strontium and barium compounds are not as decided anti-stabilizers as magnesium or calcium compounds, so that by suitably adjusting the proportions of strontium,

portion as the generation of the alkaline sta-' bilizer decomposition products, and most suitable in connection with the addition of such further cyanide as is requlredby the normal exhaustion of the bath both by reason of cementation action and by reason of adhesion losses, that is, those losses due to the adhesion of the material of the bath to iron bodies removed therefrom.

By following the teachings of this specification, case-hardening of iron bodies may be made much more eflicient. The case-hardening may be made to take place rapidly or slowly, and so as to penetrate slightly or to a great depth. The

case-hardening bath may be kept in operation for a long period of time, even to weeks, by adding thereto, at suitable intervals, the necessary compound, or compounds, or mixtures necessary in accordance with the teachings of this invention to counteract the Meet of decomposition products formed within the bath in order to keep the activity of the bath at the desired point.

By following the teachings of this specification, iron bodies may be case-hardened to a depth quite unattainable by prior procemes, especially the depth of the glass-hard layer, and with a complete absence of over-carbonization, with a gradual and uniform transition from the-glass hard layer to the unaltered iron, and with almost complete certainty.

One specific mixture forming a proper bath wouldbe a molten mixture of 50 parts barium chloride, 25 parts sodium chloride, 25 parts potassium chloride, and 5 to 10 parts of sodium cyanide. This would form a very efilcacious bath for cementation or case-hardening. 'Ihe bath, however, loses a considerable part of its desired case-hardening properties owing'to the formation of sodium oxide and barium carbonate.. Further quantities of sodium cyanide may be added to the bath, but the addition, although raising the emciencysomewhat, willpneverthe less, fail to restore the bath to its original etficiency. In accordance with applicants invention, if strontium chloride is added to the bath while there is still at least 5% of sodium cyanide present, the cementation or case-hardening action is restored to its original value.

If the cyanide content has dropped substantially below the 5% limit, then, of course, it should be brought up to, at least, that limit before expecting the strontium chloride to restore the bath to its original efliciency.

In practice, where a molten bath, as hereinbefore described, is used daily, there should be added to the mass, daily,=3% to 5% of a mixture comprising onepart strontium chloride and about 7 parts barium chloride. There can, with perhaps an increase in efficiency, also be added a little alkaline chloride.

The efficiency of the bath may be preserved or restored by the addition of barium chloride instead of strontium chloride, when this is done, however, it is necessary to add a larger amount of barium chloride than the totalfof the barium and strontium chlorides, as hereinbefore specified, for

reviving the bath.

We may make a bath comprising 70 parts barium chloride, 10 parts strontium chloride, 10 parts alkali chloride. as sodium chloride, and 10 parts sodium cyanide. A bath, of this composition, used at the temperature usually employed in cementation work, decomposes rather rapidly. In order to retard the decomposition, about 3 to 5 parts of barium oxide or from 5 to 10 parts of barium carbonate are incorporated therein. This bath would be revived daily, so that it may be brought up to the proper efliciency, or activity, by incorporating therein, daily, a mixture of the amount indicated in the previous example.

Although we have particularly described the principles of our process, and preferred ways of practicing the process, and the particular and specific mixtures of substances preferred in practicing the process, nevertheless, we desire to have it understood that the examples given are merely illustrative, but do not exhaust the possibilities of the process, or the possible substances usable in carrying out the process.

What we claim as new and desire to secure by Letters Patent of the United States, is:

l. The process of operating a case-hardening molten bath of a cyanide which consists in adding thereto a substance, selected from the group comprising alkaline earth metal chlorides and cerium chloride, neutralizing the efiect of alkaline substances formed in the bath by use, in proportion as those alkaline substances are formed.

2. The method of operating a bath for casehardening iron which includes parts barium chloride, 25 parts sodium chloride, 25 parts potassium chloride, and 5 to 10 parts sodium cyanide which consists in adding thereto, daily, 3 to 5% of a mixture of about one part strontium chloride and '7 parts barium chloride.

3. A bath for case-hardening iron which includes 50 parts'barium chloride, ,25 parts sodium chloride, 25 parts potassium chloride, at least 5 parts of sodium cyanide,-and strontium chloride.

4. A bath for case-hardening iron which includes 50 parts barium chloride, 25 parts sodium chloride, 25 parts potassium chloride, at least 5 parts of sodium cyanide and a salt of a' base more feebly electro-positive than sodium, selected from the group comprising alkaline earth metal chlo- 6. The method of regulating and maintaining the activity of a cyanide cementation bath in which reaction products formed during the process of cementation stabilize the decomposition of the cyanide, which consists in the gradual or periodic addition of anti-stabilizing agents selected from the group comprising the alkaline earth metal chlorides and cerium chloride.

WALTER BECK. KLAUS BONATH. 

